In order to reduce the landing distance of a jet engine powered aircraft, as well as to increase the margin of safety when the aircraft is landing on a wet or icy runway, thrust reversers are utilized on the jet engines in order to provide a braking thrust for the aircraft. Typically, such thrust reversers are formed by a pair of thrust reverser "doors" which are capable of pivoting between two positions about an axis which is transverse to and substantially diametrical with respect to the jet of the engine. A pair of half-shells are pivotally mounted so as to surround the reverser doors and are positioned rearwardly thereof and with the doors form the exhaust nozzle in the stowed position. When deployed, the half-shells move downstream, behind the reverser doors and out of the path of the exhaust.
The first position finds the doors in a stowed position, out of the direct path of the exhaust blast of the engine. In this position, the doors and the half-shells form the exhaust nozzle of the gas turbine engine so that the thrust of the engine is directly rearward, thereby producing the forward thrust of the aircraft. In the second position, the doors have been pivoted about the pivot axis to a transverse, blast deflecting or deployed position, to intercept and redirect the jet blast and produce the braking thrust for the aircraft when needed.
Because of the effect that the thrust reversers have on the propulsion forces, it is essential that the doors not be able to deploy except on command. To accomplish this, latching systems have been developed to provide the needed margin of safety.
For example, French patent 2,494,175 and U.S. Pat. No. 4,422,605 describe a thrust reverser system which cannot deploy unless the engine rating is close to idle. To fulfill the operating criteria of this type of thrust reverser, the unlatching sequence is the key element. These patents describe in detail that it is necessary to bring the reverser doors to a super-retracted position with respect to their folded or stowed position, in order to unlock these doors. The doors can be brought from their folded or stowed position to their super-retracted position only when the speed of the engine is lower than a certain threshold, close to idling power.
Such a system provides a relatively good protection against inadvertent, in-flight deployment, since the deployment of the thrust reverser doors can only be achieved in a restricted flight envelope. However, such a system has a particular drawback which arises in so far as safety on the ground is concerned, in having to wait until the engine speed has actually spooled down to, or close to idle, before the control actuators for the reverser doors can operate to actually bring the doors from their folded position to their super-retracted position, in order to unlatch the latches, and thereafter allow deployment of the thrust reverser.
In the normal landing mode, this operational sequence for the thrust reverser is not a problem, and indeed the thrust reverser operates precisely in the manner intended. In other words, in a normal landing mode, the engine is already spooled down close to idle speed when the thrust reverser is deployed.
The drawback arises in the case of an aborted take-off, when it is essential that the thrust reverser be capable of being deployed as quickly as possible, and particularly so on a wet or icy runway. Too much elapsed time between the command to deploy and the beginning of actual deployment sequence may very well lead to a catastrophic event, as the aircraft may run off of the runway.
A further drawback of the prior art resides in the fact that when the thrust reverser doors are brought from their stowed position to their super-retracted position, the controlling actuator(s) must counteract parasitic loads which have nothing to do with the predetermined threshold value under which the deployment sequence can be initiated. These parasitic loads, which essentially arise from the compression of the seals when the thrust reverser doors are brought from their folded or stowed position to their super-retracted position, can take as much as 40% of the available hydraulic or pneumatic pressure, leaving only 60% of the pressure to counteract the true load acting on the reverser doors.
Thus, a primary object of the present invention is to provide a latching mechanism for a thrust reverser which ensures complete operational safety of the thrust reverser by completely preventing inadvertent in-flight deployment, while allowing rapid deployment on the ground.
Another object of the invention is to provide a thrust reverser which significantly reduces the elapsed time between the command for deployment of the thrust reverser and actual deployment.